The Global Positioning System (GPS) is a satellite navigation system designed to provide position information almost anywhere in the world. GPS was developed by the Unites States Department of Defense, and currently includes a constellation of twenty-four operational satellites. Other types of satellite positioning systems include the Wide Area Augmentation System (WAAS), the Global Navigation Satellite System (GLONASS) deployed by the Russian Federation, and the Galileo system planned by the European Union.
A variety of receivers have been designed to decode the signals transmitted from the positioning satellites for the purposes of determining the position of the respective receiver on or near the Earth's surface. In order to decipher the signals and compute a final position, the receiver acquires signals from the specific satellites that are in view to the receiver, and then measures and tracks the received signals and recovers navigational data from the signals.
By accurately measuring the distance from three different satellites, the receiver can triangulate its position, e.g., solving for a latitude, longitude and altitude. The receiver measures its distance to the different satellites by measuring the time it takes for each signal to travel from the respective satellite to the receiver. Often, measurements from a fourth satellite are used to help resolve time measurement errors, e.g., errors created by the inaccuracies of timing circuits within the receiver. In some cases, signals from fewer than three satellites can be used in combination with terrestrial signals to triangulate the position of the receiver, particularly when visibility to additional satellites is limited.
GPS receivers have been implemented in subscriber units of wireless communication systems in order to allow the users of the subscriber units to exploit GPS positioning capabilities. A subscriber unit generally refers to a mobile wireless device used by an end user, such as a mobile radiotelephone.
Traditional GPS receivers can take several minutes to search and identify satellites that are in view to the receiver. Then, after identifying such satellites, the GPS receiver can compute its position. In order to accelerate the time it takes the GPS receiver in a subscriber unit of a wireless communication system to identify satellites that are in view to the receiver, GPS assistance techniques have been developed. In particular, signals detected within the wireless communication system can be used to generate a rough estimate of the location of the subscriber unit. Then, GPS assistance information can be sent to the subscriber unit in order to allow the subscriber unit to more quickly identify the satellites that are in view to its GPS receiver. In code division multiple access (CDMA) systems, for example, measurements of signals from base stations in the CDMA system are used to calculate GPS assistance information. In CDMA, the process of using measurements of base station signals to provide GPS assistance is referred to as Advanced Forward Link Trilateration (AFLT).
GPS assistance techniques can greatly accelerate the time it takes a subscriber unit to identify its location using GPS. This is particularly important when the location identification is used to help first responder services, such as the “911” first responder service, to quickly pinpoint the location of the subscriber unit so that help can be dispatched to that location. GPS assistance techniques can also improve the ability of a receiver to compute a position solution in certain locations where GPS positioning techniques might fail without the assistance, such as the interior of buildings.